13.3.2 Circuit Operation
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The speakerphone can only provide a one-way communication path.
In other words, it can either transmit an outgoing signal or receive an incoming signal at a given time,
but cannot do both simultaneously. Therefore, a switching circuit is necessary to control the flow of the
outgoing and incoming signals.
This switching circuit is contained in IC601 and consists of a Voice Detector, TX Attenuator, RX
Attenuator, Comparator and Attenuator Control. The circuit analyzes whether the TX(transmit) or the
RX(receive) signal is louder, and then it processedthe signals such that the louder signal is given
precedence.
The Voice Detector provides a DC input to the Attenuator Control corresponding to the TX signal.
The Comparator receives a TX and a RX signal, and supplies a DC input to the Attenuator Control
corresponding to the RX signal.
The Attenuator Control provides a control signal to the TX and the RX attenuator to switch the
appropriate signals on and off. The Attenuator Control also detects the level of the volume control to
automatically adjust for changing ambient conditions.
1. Transmission signal path:
The input signal from the microphone is sent through the circuit via the following path:MIC →
Pin 9 of IC601 → Pin 10 of IC601 → Pin 3 of IC601 → Pin 4 of IC601 → R601 → C602 → Q108
→ Telline.
2. Reception signal path:
Signals receive from the telephone line are outputted at the speaker via the following path:Tel
line → Q108 → Q109 → C112 → R600 → C603 → Pin 27 of IC601 → Pin 26 of IC601 → Pin19
of IC601 → Pin 15 of IC601 → Speaker.
3. Transmission/Reception switching
The comparison result between TX and RX outputs as a DC level of Pin 25 of IC601.TX level is
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